Polypropylenes, which have many desirable physical properties, have been applied to a wide variety of uses. For example, they have been widely used in the field of packaging films. In this field of use they are generally supplied as propylene random copolymers in which propylene is randomly copolymerized usually with ethylene and/or an .alpha.-olefin other than propylene in order to improve the heat sealability at low temperatures. Such propylene random copolymers have a low crystallinity, low melting point, high transparency and high heat sealability attributable to highly random copolymerization of the comonomers. However, they have a problem of remarkable increase of solubility to organic solvents (e.g. saturated hydrocarbon solvents), which may threaten the safety of foods, with the increase of the content of ethylene and/or an .alpha.-olefin other than propylene.
On the other hand, there has been known a method for improving the blocking resistance and mechanical strength of propylene random copolymer compositions by modifying a random copolymer of propylene and an .alpha.-olefin other than propylene by incorporating thereinto 5-50% by weight of an isotactic polypropylene (JP-A-54-48846 and JP-A-54-95684). However, the method merely serves to keep the level of transparency and heat sealability at low temperatures. It is still unsatisfactory to meet the requirement to provide a propylene random copolymer composition with a good balance between solvent resistance and transparency. Specifically, increasing the proportion of isotactic polypropylene for improving the solvent resistance broadens the distribution of the composition and adversely affects the transparency. Use of random copolymers of low crystallinity alone for improving the transparency, in turn, notably reduces the solvent resistance. Thus, the method cannot solve the problem underlying the present invention.
There have also been known various methods for improving the transparency of propylene random copolymer compositions by adding thereinto any nucleating agent (JP-A-59-164348, JP-A-62-132937 & JP-A-2-51548). However, nucleating agents are generally expensive due to their complicated structure and technical difficulty in production. Therefore, these methods are not industrially practical in view of the cost factor.